As energy prices are increasing due to depletion of fossil fuels and interest in environmental pollution is escalating, the demand for environmentally-friendly alternative energy sources is bound to play an increasing role in future. Thus, research into techniques for generating various powers, such as nuclear energy, solar energy, wind energy, and tidal power, is underway, and power storage devices for more efficient use of the generated energy are also drawing much attention.
In particular, the demand for lithium secondary batteries as energy sources is rapidly increasing as mobile device technology continues to develop and the demand for the mobile devices continues to increase. In recent years, use of lithium secondary batteries as a power source of electric vehicles (EVs) and hybrid electric vehicles (HEVs) has been realized, and the market for lithium secondary batteries continues to expand to applications such as auxiliary power suppliers through smart-grid technology.
A lithium secondary battery has a structure in which an electrode assembly, in which a porous polymer film is interposed between a positive electrode and a negative electrode, each of which includes a coating layer applied to a current collector, is impregnated with a lithium salt-containing non-aqueous electrolyte. A lithium cobalt-based oxide, a lithium manganese-based oxide, a lithium nickel-based oxide, a lithium composite oxide, or the like is mainly used as a positive electrode coating layer. A carbon-based material is mainly used as a negative electrode coating layer.
However, in a lithium secondary battery using a carbon-based material as a negative electrode coating layer, irreversible capacity occurs in some lithium ions intercalated into a layered structure of the carbon-based material during a first charging and discharging cycle with the result that discharge capacity is reduced. In addition, a carbon material have a low oxidation/reduction potential of about 0.1 V with respect to potential of Li/Li+ with the result that a non-aqueous electrolyte decomposes at the surface of the negative electrode, and the carbon material reacts with lithium to form a layer coated on the surface of the carbon material (a passivating layer or a solid electrolyte interface (SEI) film). The thickness and boundary states of such an SEI film vary according to an electrolyte system used and thus affect charge and discharge characteristics. Furthermore, in a secondary battery used in a field, such as a power tool, which requires high output characteristics, resistance increases due to such an SEI film having a small thickness with the result that a rate determining step (RDS) may occur. In addition, a lithium compound is produced at the surface of the negative electrode with the result that the reversible capacity of lithium gradually decreases as charging and discharging are repeated, and therefore discharge capacity is reduced and cycle deterioration occurs.
Meanwhile, the use of a lithium titanium oxide (LTO) as a negative electrode coating layer having structural stability and good cycle characteristics is under consideration. In a lithium secondary battery including such a LTO as a negative electrode coating layer, a negative electrode has a relatively high oxidation/reduction potential of about 1.5 V with respect to potential of Li/Li+ with the result that decomposition of an electrolyte hardly occurs, and excellent cycle characteristics are obtained due to stability of a crystal structure thereof.
In addition, LiCoO2 has been mainly used as a positive electrode coating layer. At present, however, an Ni-based material (Li(Ni—Co—Al)O2), an Ni—Co—Mn-based material (Li(Ni—Co—Mn)O2), and a high-stability spinel type Mn-based material (LiMn2O4) are used as other layered positive electrode coating layers. In particular, a spinel type manganese-based battery was applied to a mobile phone for a while. However, the spinel type manganese-based battery disappeared from the market of high-function mobile phones due to the reduction in energy density of the spinel type manganese-based battery although the spinel type manganese-based battery was not expensive. For this reason, much research has been conducted into methods of increasing energy density of a spinel type manganese-based positive electrode active material.
Several methods of increasing energy density of a lithium secondary battery may be considered. Among them is a method of increasing an operating potential of the battery, which is effective. Conventional lithium secondary batteries using LiCoO2, LiNiO2, and LiMn2O4 as a positive electrode coating layer have an operating potential of 4 V level. The average operating potential of lithium secondary batteries is 3.6 to 3.8 V. This is because the potential is decided according to oxidation and reduction of Co ions, Ni ions, or Mn ions. On the other hand, in a case in which a compound having a spinel structure in which some of Mn of LiMn2O4 is replaced with Ni, etc. is used as a positive electrode coating layer, it is possible to obtain a lithium secondary battery having an operating potential of 5 V level. In recent years, therefore, a lithium nickel manganese oxide (LNMO) has been examined as a positive electrode coating layer corresponding to a negative electrode coating layer made of a lithium titanium oxide.
However, in a lithium secondary battery including a lithium titanium oxide and a lithium nickel manganese oxide as a positive electrode coating layer and a negative electrode coating layer, the total energy and life stability of the lithium secondary battery are affected by the coating areas and arrangement of a positive electrode and a negative electrode. For this reason, it is important to provide proper coating areas and arrangement of the positive electrode and the negative electrode.
Therefore, there is a high necessity for technology that is capable of fundamentally solving the above problems.